Dialysis treatment of individuals suffering from renal failure requires that the blood be withdrawn and cycled through a dialysis machine that performs the function of the failed kidneys. This process, termed hemodialysis, must be repeated periodically and thus requires repeated puncture wounds using a dialysis needle. Moreover, dialysis requires a relatively rapid blood flow rate, typically above 200 ml/min, and so the dialysis needle is relatively large. Host vessels have insufficient strength to withstand collapse from such frequent puncturing with large bore needles.
A common technique to provide vascular access for hemodialysis, therefore, is to connect a prosthetic arteriovenous (AV) graft or shunt between an artery and a vein in, for example, the arm. The AV graft is constructed to withstand numerous puncture wounds or "sticks" without collapse.
Conventional AV grafts are typically constructed of woven or knitted polyethylene terepthalate (PET). Unfortunately, conventional AV grafts must be implanted for at least two weeks prior to puncture so that an intimal layer of fibrotic tissue has an opportunity to attach to the luminal surface of the graft. The layer of fibrotic tissue prevents blood leakage through the wall of the graft upon puncture. Prior to the time at which the graft can be safely punctured without leakage, a central venous catheter (CVC) must be utilized to collect the blood required for cycling through the dialysis machine. The CVC is needed because of the relatively high blood flow rates involved. For certain patients, however, use of a CVC is contraindicated.
Various attempts at designing a vascular access graft that will not leak if punctured immediately after implant have been made. One such graft is seen in the U.S. Pat. No. 4,619,641, in which the graft has two expanded polytetrafluoroethylene (PTFE) tubes in coaxial relationship with a space of about 1 mm therebetween filled with a self-sealing elastomer, such as silicone. Silicone often tends to stiffen the graft which is undesirable when trying to shunt between two fairly closely spaced vessels. In addition, silicone may have a tendency to exude inward through the puncture hole in the wall of the graft and therefore occlude the lumen.
Both U.S. Pat. Nos. 5,116,360 and 5,700,287 disclose vascular access grafts that ostensibly seal around puncture wounds. These two patents utilize various layers of fibers or other materials to slow the blood flow through the wall of the graft and cause its clotting.
Although the prior art includes many different designs of self-sealing vascular access grafts, none has proved effective in sealing around a puncture wound immediately after implant of the graft. Instead, grafts of the prior art exhibit excessive leakage or occlusion of the lumen. In some instances, occlusion of the graft lumen becomes so severe that the blockage within the graft must be removed in a process known as "revising" the graft. The procedure typically involves clamping the inflow end of the graft, making an incision to access the graft interior, clearing the block, and sewing the graft incision closed. Unfortunately, some self-sealing grafts are constructed in a manner that results in excessive fraying or layer dissection when they are incised, thus unduly lengthening or complicating the revision process.
Another drawback with some self-sealing grafts is their bulky construction that interferes with sensing of blood pressure pulsation. That is, as with a conventional needle stick of a natural vessel, the medical personnel establishing a dialysis circuit must "find" the graft under the skin. Searching for a pulse is one means of finding a vessel to be accessed, and thus excessive structure in some self-sealing grafts that attenuates the blood pressure pulses makes the search for the graft that much harder. Despite this drawback with thick-walled self-sealing grafts, the prior art has tended in the direction of more rather than less layers or barriers between the blood flow lumen and the graft exterior, under the theory that such layers or barriers enhance the goal of inducing a clot around a needle access site. Whether this theory works or not, the more layers or barriers the more attenuated is the blood pulse through the graft wall.
Because of the drawbacks associated with prior vascular access grafts, there is a need for an improved vascular access graft that enables rapid puncture immediately after implantation and resists collapse or lumen occlusion from repeated needle punctures.